RECENT EXPERIMENTS TOUCHING NUCLEON DRIP LINES AND NEW REGIONS OF DEFORMATION

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RECENT EXPERIMENTS TOUCHING NUCLEON DRIP LINES AND NEW REGIONS OF

DEFORMATION

S. Hoath

To cite this version:

S. Hoath. RECENT EXPERIMENTS TOUCHING NUCLEON DRIP LINES AND NEW RE- GIONS OF DEFORMATION. Journal de Physique Colloques, 1986, 47 (C4), pp.C4-223-C4-228.

�10.1051/jphyscol:1986426�. �jpa-00225792�

RECENT EXPERIMENTS TOUCHING NUCLEON DRIP LINES AND NEW REGIONS OF DEFORMATION

Department of Physics, University of Birmingham, P.O.B. 363.

GB-Birmingham B15 2TT, Great-Britain

A b s t r a c t - I m p o r t a n t n u c l e a r s t r u c t u r e i n f o r m a t i o n a l r e a d y g a i n e d f r o m e x p e r i m e n t s i n t h e i n t e r m e d i a t e e n e r g y r e g i m e is summarised. The d i r e c t i o n a n d l i k e l y s u c c e s s o f f u t u r e s t u d i e s o f e x o t i c n u c l e i are a l s o d i s c u s s e d , e s p e c i a l l y f o r t h e n u c l e o n d r i p l i n e s and new r e g i o n s of d e f o r m a t i o n .

I - INTRODUCTION

Most o f o u r s e m i - e m p i r i c a l n u c l e a r s t r u c t u r e t h e o r y a r i s e s f r o m s t a b l e n u c l e i , w h i c h a l l h a v e r a t h e r s i m i l a r A/Z r a t i o s a n d g e n e r a l l y l o w s p i n a n d e x c i t a t i o n . We m u s t t h e r e f o r e e x p e c t t o s e e r a t h e r s i m i l a r p h y s i c s down i n t h e v a l l e y o f s t a b i l i t y / I / . T h o s e who p r e f e r t o c l a m b e r u p t h e s i d e s s e e k t o f i n d some r a r i t y o r t o r e a c h t h e p l a t e a u a b o v e f o r a r a t h e r d i f f e r e n t a n d more g l o b a l v i e w . T h e r e are some d a n g e r s t o t h i s a p p r o a c h , f o r example i n p a r t i c l e p h y s i c s where t h e s t r u g g l e t o w a r d s t h e t o p ( q u a r k ? ) n e c e s s a r y t o a c M e v e t h e u n i f i c a t i o n o f t h e 100GeV i n t e r m e d i a t e v e c t o r b o s o n s w i t h t h e m a s s l e s s p h o t o n may h a v e r e v e a l e d a ' d e s e r t ' / 2 / . By c o n t r a s t t h e u p r i s i n g e f f o r t s a r o u n d t h e F e r m i domain i n Normandy p r o v i d e a r e a l ' d e s s e r t ' f o r u s a l l

....

("present address : EDWARDS HIGH VACUUM, Vacuum Training Centre, Manor Royal, GB-Crawley RHlO 2LW.

West Sussex. Great-Britain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986426

JOURNAL DE PHYSIQUE

mechanisms responsible for this welcome bounty, I shall present some of the more important nuclear structure results and the future understanding that is

anticipated from the exploration of the Fermi energy regime at GANIL and similar laboratories.

I1 - NUCLEON DRIP LINES,

Adding many neutrons to a light isotope, and studying the evolution of the resultant level spectra, is one classic approach to extending our knowledge of nuclear forces and their symmetries. The single particle shell model for spherical nuclei predicts N = 20 is a magic number, but in the light elements around sodium there is evidence that the nuclear ground states are quite highly deformed. An examination of the nuclear masses, lifetimes and energy spectra of such light 'exotics' is of especial interest.

In order to know the range of beta-decaying nuclei that might be available for such studies, the limits of nuclear stability against nucleon(s) emission need to be established. This has been one aim of the first generation work by the Orsay-GANIL PE46 collaboration over the last 2 years. With the end-products of heavy ion bombardment of various targets analyzed at 0 degrees with LISE observations of both neutron-rich and neutron-deficient nuclei have been made for the first time. Reasonable criteria for particle instability can be

formulated and applied to establish the non-existence of isotopes too / 4 / , and it seems that the so-called 'nucleon drip linesq are now reached for proton rich nuclei up to calcium (T = -5/2 series) and for neutron rich nuclei up to nitrogen (23~).

I11

-

In the interesting N : 20 region, several experiments have searched for 260, 2 9 ~ and 3 2 ~ e but so far without success. This may reflect both the reaction mechanisms and the beam energy ceiling imposed by the 3.2Tm rigidity limit of LISE for such massive low,Z 'exotics' rather than the nuclear structure. A mass measurement 29y30~e would be well worthwhile and would complement very recent /9/ work done elsewhere at lower energies on the N = 20 isotones 3 5 ~ , 3 4 ~ i and 3 3 ~ 1 and the N : 21 nucleus 3 5 ~ i .

For beta-decaylng exotic nuclei produced at Fermi energies it is possible to determine their inertial mass by measurement of their velocity and momentum.

The overall mass measurement accuracy required to test the predicted 'drip lines' is less than 0.5MeV, which is quite difficult t;o achieve for particles with about 1GeV kinetic energy. Despite this, some very elegant studies using the SPEG magnetic spectrometer at GANIL have recently emerged for nuclei very close to the

Sulphur

v,

!- Z

2

U

1

23 25 2 7 A

2 1 29 31 A

Flg. 1 Evldence for observat~on of the Tz = -512 serles: 3 5 ~ a , 3 1 ~ r , 2 7 ~ and 2 3 ~ 1 .

(Langevln et a1 / 4 / , Nuclear Physlcs A (1986) to be published.)

Fig. 2 Evidence for stability of '*c and 1 9 ~ against nucleon emission

.

( Paugheon et a1 / 4 / . )

2

3

1 0 ' -

1 -

Carbon

I!? , ,

17 18 e 20 zr ^{22 23} A

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l i m i t o f s t a b i l i t y : 2 0 ~ , 230, 2 4 9 2 5 ~ s o f a r ( B i a n c h i e t a 1 t o b e p u b l i s h e d ) . Mass d e t e r m i n a t i o n s o f o r o t o n r i c h e x o t i c s c o u l d b e u s e d t o e x a m i n e t h e T 2

i s o s p i ~ m u l t i p l e t s i n l i g h t n u c l e i , b u t would b e b e t t e r c o n s t r u c t e d

e x p e r i m e n t a l l y v i a d e c a y e n e r g e t i c s . P r o g r e s s t o w a r d s t h e e q u a l l y d i f f i c u l t i d e n t i f i c a t i o n o f a n a l o g u e s a t 20MeV e x c i t a t i o n i n t h e v a l l e y i t s e l f h a s b e e n a c h i e v e d f o r t h e A = 2 3 s y s t e m / l o / , a n d s h o u l d prompt f u r t h e r ' e f f o r t s on t h e e x o t i c s .

B e t a d e c a y l i f e t i m e s a n d p r e f e r a b l y l e v e l s c h e m e s would b e m o s t u s e f u l c h e c k s on n u c l e a r s t r u c t u r e b e c a u s e t h e y g i v e more d i r e c t tests o n t h e w a v e f u n c t i o n s a n d o v e r l a p s b e t w e e n i n i t i a l a n d f i n a l states. T h i s h a s r e c e n t l y been a c h i e v e d by w o r k e r s a t MSU a s r e p o r t e d a t t h i s c o n f e r e n c e f o r t h e l i g h t e r e x o t i c n u c l e i

1 4 ~ e a n d 1 7 ~ / 1 1 / . U n f o r t u n a t e l y ' e x o t i c ' l e v e l s c h e m e s w i l l b e o n l y m e a s u r a b l e f o r d a u g h t e r s o f r a d i o a c t i v e ' e x o t i c ' p a r e n t n u c l e i : t h e s p e c t r o s c o p y o f t h e m o s t a c c e s s i b l e n u c l e i h a s r e c e n t l y b e e n d e t e r m i n e d on LISE a t GANIL u s i n g t e c h n i q u e s

( e . g . / 1 2 / ) o f e n h a n c e d i s o t o p e s e l e c t i o n p r o v i d e d by e n e r g y d e g r a d e r s . R e s u l t s w i l l b e r e p o r t e d t o t h i s c o n f e r e n c e f o r 4 0 ~ , 37,3BP, 3 5 , 36si, 26Ne, 2 4 F , 220

,"

I I

a n d C / 1 3 / .

F u r t h e r work t o get t o t h e p a r e n t l e v e l s c h e m e s must r e l y o n h i g h f l u x s e c o n d a r y beams / 1 4 / u s i n g i n e l a s t i c s c a t t e r i n g t e c h n i q u e s o r t h e a p p l i c a t i o n o f gamma r a y s h r o u d s ( e . g . t h e UK D a r e s b u r y L a b o r a t o r y ' s POLYTESSA a r r a y / 1 5 / ) i n m u l t i p l e c o i n c i d e n c e w i t h i n t e r m e d i a t e e n e r g y b e a m l l i n e s l i k e LISE. B o t h t h e s e o p t i o n s a r e v e r y h a r d e x p e r i m e n t a l c h a l l e n g e s as shown by some c r u d e r a t e c a l c u l a t i o n s .

I V - FUTURE OUTLOOK

A s t h e r e is more i n t e r e s t i n t h e l e v e l s o f f o r example 2 2 ~ and 240 t h a n 2 2 ~ a n d 2 4 ~ , b e c a u s e t h e tests o f n u c l e a r d e f o r m a t i o n are s i m p l e r t o u n r a v e l f o r t h e e v e n - e v e n i s o t o p e s , t h e r e w i l l b e c o n t i n u e d p r e s s u r e t o make t h e s e p a r e n t s a s e f f i c i e n t l y a s p o s s i b l e . T h i s e n c o u r a g e s u s t o t a k e care t h a t t h e e x c i t a t i o n e n e r g y p u t i n t o t h e p r i m a r y n u c l e i a t Fermi e n e r g i e s d o e s n o t g e t s o l a r g e t h a t t h e d e t e c t e d r e s i d u a l s a r e r a t h e r less e x o t i c t h a n e x p e c t e d /a/.

T h e r e w a r d f o r c a r e f u l p r e p a r a t i o n s o f o u r e x p e r i m e n t a l e x o t i c s p r o d u c t i o n l i n e w i l l b e t o c h a r t d e f o r m a t i o n s i n t h e l i g h t e s t n u c l e i . C a l c u l a t i o n s b a s e d o n H a r t r e e - F o c h - B o g l i u b o v t h e o r y h a v e b e e n made r e c e n t l y a t O r s a y / 1 6 / f o r e x o t i c n e o n a n d oxygen i s o t o p e s .

I n c o n t r a s t t h e r e are a l r e a d y p u b l i s h e d s h e l l model p r e d i c t i o n s f r o m t h e U t r e c h t g r o u p / I T / e x p l i c i t l y f o r 1-p s h e l l e x o t i c s , a n d a l s o t h e u n i v e r s a l s-d s h e l l i n t e r a c t i o n o f W i l d e n t h a l c a n b e a p p l i e d f o r h e a v i e r s y s t e m s .

Fig. 3 Touching nucleon drip lines in the light nuclei: parts of the Segre chart are shown following the recent GANIL experiments /4/.

Intense beams from ECR ion sources for gaseous and metallic elements can be exploited by choosing the accelerated isotope to optimise the rates of exotic species at the final detectors. The use of N : Z beams rather than enriched isotopes may be warranted on cost grounds too, and especially for the production of proton-rich exotics. For 2p radioactivity searches, for example from 3 1 ~ r , a beam of 3 6 ~ r could be easier than the 4 0 ~ a used initially at GANIL.

The region of deformation around N = 20 has yet to yield up all its secrets.

Information from exotic transfer reactions (Chapman et al, to be published, and Fifield et at, to be published) at lower energies is consistent with a good shell closure at N = 20 for groundstate nuclei with Z : 14 to 16. Thus until the lower Zs are probed at Fermi energies, which will be d&fficult as remaining isotopes to search for are 3 3 ~ 1 and the A/Z 3 nuclei 26, 2EiO , F'* and 3 2 ~ e , the known region of deformation remains just 3 1 ~ a and 3 2 ~ g in a sea of near-sphericity.

I would like to thank all the members of the Orsay - GANIL PE46 collaboration, with whom I worked last year on the first generation of LISE experiments, and I acknowledge financial support from IPN Orsay.

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